code fix (#13365)

paddle/fluid/operators/softmax_with_cross_entropy_op.cu

@@ -31,7 +31,8 @@ __global__ void CrossEntropyGrad(T* logit_grad, const int64_t* labels,
   for (int i = blockIdx.x * blockDim.x + threadIdx.x; i < batch_size;
        i += blockDim.x * gridDim.x) {
     int idx = i * class_num + labels[i];
-    logit_grad[idx] -= static_cast<T>(1.);
+    logit_grad[idx] -=
+        ignore_index == labels[i] ? static_cast<T>(0.) : static_cast<T>(1.);
   }
 }